Nuclear reactor for breeding U233

ABSTRACT

A light-water-cooled nuclear reactor capable of breeding U 233  for use in a light-water breeder reactor includes physically separated regions containing U 235  fissile material and U 238  fertile material and Th 232  fertile material and Pu 239  fissile material, if available. Preferably the U 235  fissile material and U 238  fertile material are contained in longitudinally movable seed regions and the Pu 239  fissile material and Th 232  fertile material are contained in blanket regions surrounding the seed regions.

CONTRACTUAL ORIGIN OF THE INVENTION

The invention described herein was made in the course of, or under, acontract with the UNITED STATES ATOMIC ENERGY COMMISSION.

This is a division, of application Ser. No. 486,786, filed July 9, 1974,now U.S. Pat. No. 3,960,655.

BACKGROUND OF THE INVENTION

This invention relates to a nuclear reactor. In more detail, theinvention relates to a reactor which is useful for breeding uranium-233for use in a light-water breeder reactor.

The design of light-water reactors capable of self-sustained breedinghas been extensively studied at Bettis Atomic Power Laboratory. Thesedesigns all include a heterogeneous array of rods fueled with (U²³³,Th)O₂ pellets with a moderator and coolant of light water. The designsalso have as a common feature the use of a movable fuel region coupledwith some form of axial fuel variation to attain reactivity control. Onesuch design is described in detail in patent application Ser. No.461,475, filed Apr. 16, 1974, now U.S. Pat. No. 3,957,575.

Before any large-scale development of (U²³³, Th)O₂ reactors can occur,it is necessary to develop an efficient means of obtaining largequantities of the fissile isotope U²³³. The only method by which U²³³can be obtained in quantity is by the conversion of Th²³² to U²³³. Thisrequires the development of an efficient reactor which is fueled withavailable fuel and in which conversion of Th²³² to U²³³ can be carriedout. One such reactor which has been proposed is a reactor which usesonly plutonium fissile fuel but operation of such a reactor is totallydependent on fissile fuel which must also be produced in a reactor.Another such reactor uses U²³⁵ as fuel with a low-cost diluent such asZrO₂ but such a reactor has a low thorium conversion ratio. Stillanother such reactor forms the subject matter of the presentapplication.

SUMMARY OF THE INVENTION

According to the present invention, breeding of U²³³ for use in alight-water thermal reactor capable of self-sustained breeding isaccomplished in a reactor fueled with U²³⁵ with U²³⁸ diluent and Pu²³⁹fuel, if available, plus Th²³² diluent. The fertile material Th²³² isphysically separated from the U²³⁵ and is preferably mixed with Pu²³⁹ toimprove Th²³² conversion to U²³³. A preferred design includeslongitudinally movable seed regions containing the U²³⁵ and U²³⁸ whichare surrounded by Pu²³⁹ -Th²³² blanket regions. The seed material iscapped with natural UO₂ and the blanket fuel with Th²³² O₂ to reduceneutron leakage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical section of a nuclear reactor incorporating thepresent invention,

FIG. 2 is a horizontal section thereof taken through the reactor core,

FIG. 3 is an enlarged horizontal section taken through a single fuelmodule,

FIG. 4 is a schematic of a fuel rod, and

FIG. 5 is a graph showing the critical positions of the movable seed ofthe pre-breeder design of the present invention as well as for a typicallight-water breeder reactor as a function of core lifetime.

DESCRIPTION OF THE INVENTION

The primary purpose of the pre-breeder reactor of the present inventionis to produce fissile U²³³ for use in a breeder reactor. A reactor whichaccomplishes this is characterized by the following design parameters:

1. The U²³⁵ -enriched regions of the core should be designed such thatthe initial hydrogen-to-U²³⁵ atomic ratio is between 10.0 and 150.0 andthe initial U²³⁵ to U²³⁸ ratio is between 0.005 and 0.20.

2. The plutonium-enriched regions of the core should be designed suchthat the initial plutonium-to-hydrogen atomic ratio is between 0.0 and0.015 and the equilibrium ratio is between 0.002 and 0.015. The initialplutonium-to-Th²³² ratio should be between 0.0 and 0.03 and theequilibrium ratio between 0.005 and 0.03.

3. The binary fuel compositions shall occupy from 30 to 95% of the corevolume.

4. There will initially be at least two distinct fuel compositions. Onecomposition shall contain thorium, possibly mixed with plutonium, and atleast one composition shall contain a mixture of uranium with U²³⁵ asthe principal fissile isotope and U²³⁸ as the diluent.

5. The fuel materials may be composed of (a) uranium or plutonium metalor metal alloys, (b) uranium or plutonium oxide with or without diluentsof uranium oxide, thorium oxide, beryllium oxide or other low crosssection oxides, (c) uranium or plutonium carbide with or without adiluent of uranium carbide, thorium carbide or a low cross sectioncarbide diluent.

6. Whenever plutonium fuel is being used the isotopic mix may vary asthe sources vary, but each plutonium composition must contain at least60% of the fissile isotopes Pu²³⁹ or Pu²⁴¹.

With the use of both U²³⁵ --U²³⁸ and Pu²³⁹ --Th²³² fuels, the reactordescribed herein may be employed under a wide variety of economic ortechnical constraints. If plutonium is not available in largequantities, then it would be possible to operate for a cycle with noplutonium in the thorium. In a later cycle the plutonium produced fromthe U²³⁵ of the first cycles could be used in the thorium fuel toimprove conversion performance. Conversely, if a large amount ofplutonium were available and the criteria of prime importance were theefficient utilization of plutonium already available rather thanefficient over-all fuel utilization, then more plutonium could be usedfrom the beginning of the first cycle of the pre-breeder to accelerateits conversion into a breeder. Thus, the invention describes apre-breeder concept which is viable in almost any future economicenvironment.

As shown herein, a combination of U²³⁵ and Pu²³⁹ as fissile materialsand Th²³² and U²³⁸ as fertile materials can be employed in such a waythat

1. power comparable to a breeder is achieved

2. lifetime comparable to a breeder is achieved

3. conversion of Th²³² to U²³² is achieved

4. some amount of makeup plutonium is achieved

5. the materials are produced in core regions such that only chemicalseparation of materials is required

6. The design of the pre-breeder is mechanically the same as that of thelight-water breeder reactor mentioned hereinbefore and

7. the use of different combinations of fuels can result in differentfuel utilizations over a range of economic conditions.

The advantages of this design are:

1. The invention provides an efficient method of generating U²³³ for usein light-water breeder reactors.

2. Fissile plutonium is used in the thorium fuel to enhance U²³³conversion but the amount of plutonium used may be varied to strike anybalance desired between over-all fuel utilization and the time requiredto achieve a full breeder reactor.

3. The invention provides a useful market for the plutonium currentlybeing produced by light-water reactor plants without being dependent onthat external plutonium source.

4. Because of its flexible plutonium usage, the invention can beeconomically optimized over a wide range of economic environments.

5. The invention has the capability of being used as a replacement corein existent light-water reactor plants.

6. Mechanically, the invention is of essentially the same design aslight-water breeder reactors, simplifying conversion to breeding.

To demonstrate the ability of the uranium-plutonium pre-breeder conceptto achieve its goal of a high conversion ratio while producing asignificant amount of power and attaining a reasonable lifetime, atypical design will be described. The typical design discussed herein isincluded only for illustrative purposes. It is not the only possiblepre-breeder design, nor is it an optimal design since the possibility ofa reduction in the fissile fuel loading or an improved conversion ratioexists.

As shown in the drawing, a nuclear reactor incorporating the presentinvention comprises a core 10 enclosed within a pressure vessel 11provided with a closure head 12. Core 10 is disposed within core cageassembly 13 which includes upper core barrel 14, lower core barrel 15and bottom plate assembly 16. Inlet baffle shield barrel 17 is disposedbetween lower core barrel 15 and pressure vessel 11 and terminates ininlet flow baffle 18 having a plurality of openings 19 therein whichallow coolant to circulate therethrough. Also forming a part of corecage assembly 13 is holddown barrel 20 which is inside of upper corebarrel 14 and prevents upward movement of the reflector due to waterflowing upwardly therethrough. The upper section of holddown barrel 20and upper core barrel 14 contain slots 21 to provide for coolant exitfrom the core and to provide for differential thermal expansion.Pressure vessel 11 is provided with four inlet nozzles 22 communicatingwith a lower plenum 23 below and outside of baffle 18 and four outletnozzles 24 communicating with an upper plenum 25 which communicates withslots 21.

As more particularly shown in FIG. 2, core 10 includes a plurality offuel modules 26, each including a hexagonal seed assembly 27 surroundedby a hexagonal annular blanket assembly 28. Reflector modules 29surround blanket assemblies 28 to complete the core 10. Seed assemblies27 are moved longitudinally with respect to blanket assemblies 28 bycontrol mechanisms 29a (FIG. 1) to control the reactor. Blanketassemblies 28 are supported by support tubes 30 which are suspended fromclosure head 12 and seed assemblies 27 are supported by the controlmechanism 29a through lead screws (not shown).

Further details of the mechanical structure of this reactor will not bedescribed because they form no part of the present invention. For thesedetails reference is made to U.S. Pat. No. 3,957,575, supra.

Each seed assembly 27 includes a plurality of parallel, elongated fuelrods 31 disposed in triangular array within an hexagonal shell 32 whichseparates coolant flow in the seed from coolant flow in the blanket.Each blanket assembly 28 includes a plurality of parallel, elongatedfuel rods 31 disposed in triangular array in an hexagonal annulus arounda seed assembly 27. Six rows of blanket fuel rods are required for thedesign specifically described herein. The fuel rods 31 in the blanketare supported by a blanket support tube 33 disposed interiorly of theblanket assembly which also serves to define a channel within which theseed assembly moves. These blanket fuel rods are restrained by a cagestructure including corner posts 34 surrounding the blanket assembly asno shell is needed around the blanket assembly.

As shown in FIG. 4, fuel rods 31 include an active portion 35 and axialblanket portions 36 and contain a plurality of fuel pellets 37 stackedinside of a hollow thin-walled cladding tube 38. The fuel rods in a seedassembly are 0.306 inch O.D. and the fuel rods in a blanket assembly are0.571 inch O.D. The combined length of the active portion and axialblanket portions of a fuel rod 31 is 102 inches.

The specific fuel loading employed in this embodiment of the inventionemploys oxide pellets of fuel making use of U²³⁵ as the fissile fuel inthe seed fuel rods with U²³⁸ as fertile material and Pu²³⁹ as fissilefuel in the blanket fuel rods with Th²³² as fertile material. The axialend blankets for the seed fuel rods contain natural uranium dioxide andthe axial end blankets for the blanket fuel rods contain Th²³² O₂.

Computation of the composition of fuel needed in the seed and blanketfuel rods was carried out assuming that the fissile-bearing fuel was 97%theoretical density and that the isotopic mix of the plutonium fuel wasPu²³⁹ 78%, Pu²⁴⁰ 12%, Pu²⁴¹ 10% plus a negligible trace of Pu²⁴². Thecomposition of the active portion of the fuel and blanket rods will nextbe given. The upper third of the active portion of the seed fuel rods is1.458 weight percent U²³⁵, the middle third is 2.914 weight percent U²³⁵and the lower third is 10.194 weight percent U²³⁵.

Weight percent U²³⁵ is defined as ##EQU1##

The active portion of the blanket fuel rods contains plutoniumdistributed as follows: The innermost row of blanket fuel rods 31contains 0.473 weight percent Pu²³⁹, the second row contains 6.947weight percent Pu²³⁹ in the upper five-sixths thereof and 0.473 weightpercent Pu²³⁹ in the lower one-sixth thereof and the remaining four rowscontain 2.362 weight percent Pu²³⁹ in the upper one-third thereof, 1.655weight percent Pu²³⁹ in the next 1/6 thereof, 0.947 weight percent Pu²³⁹in the next 1/3 thereof and 0.473 weight percent Pu²³⁹ in the bottom 1/6thereof.

Weight percent Pu²³⁹ is defined as ##EQU2##

The fissile fuel loading of the reactor is 22 kilograms of U²³⁵ in theseed region and 10 kilograms of Pu²³⁹ in the blanket region of eachmodule.

The critical positions of the movable seed of the pre-breeder design areshown on FIG. 5 as a function of core lifetime. For comparativepurposes, FIG. 5 also shows the critical positions for a typicallight-water breeder reactor. Since the two designs are at the same powerlevel, FIG. 5 indicates that the pre-breeder design is capable ofessentially the same power lifetime at an LWBR design.

In discussing the effectiveness of the typical pre-breeder design somefigure of merit concerning the production of U²³³ must be defined. Themain parameter of interest is the amount of U²³³ produced per netfissile atom lost over the entire core, which is defined at any time inlife as: ##EQU3## where gain and loss at any time are measured from timeO and (B) indicates the isotope is in the plutonium-thorium region and(S) indicates the U²³⁵ --U²³⁸ region. The isotope Pa²³³ is counted asU²³³ because it will decay to U²³³ with a 27.4-day half-life whenremoved from the core.

The table below is a partial mass balance for the typical design at 4times in life which also shows the appropriate C values. The value of Cat 15,000 EFPH is 0.641. For this typical design, with assumed 1%fabrication and reprocessing losses, 5.21 pre-breeder cycles of 15,000EFPH each are required to produce enough U²³³ for a light-water breederreactor requiring 31.65 kg per module. Since in practice a full 6 cycleswould probably be run, it would require 10.3 years to achieve alight-water breeder, but an additional 13% of 4.11 kg of U²³³ would havebeen produced.

    __________________________________________________________________________     TYPICAL PRE-BREEDER FISSILE FUEL MASS BALANCE                                __________________________________________________________________________           Effective Full                                                                Power Hours                                                                            0      3000   9000   15,000                                   __________________________________________________________________________           U.sup.233                                                                              .0108  .0095  .0079  .0067                                    SEED   U.sup.235                                                                              22.041 20.943 18.569 16.381                                   (Movable                                                                             Pu.sup.239                                                                             0      .6044  1.3943 1.9167                                   Region)                                                                              Pu.sup.242                                                                             0      .0115  .0882  .2014                                           Pa.sup.233                                                                             0      .4886  .4954  .4816                                    BLANKET                                                                              U.sup.233                                                                              0      1.0984 3.6276 5.5863                                   (Stationary                                                                          U.sup.235                                                                              0      .0011  .0118  .0358                                    Region)                                                                              Pu.sup.239                                                                             9.8390 7.9910 5.4596 3.7094                                          Pu.sup.241                                                                             1.2720 1.3175 1.4172 1.4380                                          C        --     0.695  0.664  0.641                                    __________________________________________________________________________

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. Method of producing U²³³for use in a light-water-cooled breeder reactor comprising operating aseed-and-blanket reactor containing U²³⁵ fuel and U²³⁸ fertile materialin the seed and Th²³² fertile material in the blanket, the Th²³² beingphysically separated from the uranium, to produce Pu²³⁹ in the seed andU²³³ in the blanket, recovering the Pu²³⁹ and U²³³ thus produced,incorporating the Pu²³⁹ thus produced in the blanket of aseed-and-blanket reactor along with Th²³² wherein the seed consists ofU²³⁵ and U²³⁸, operating the reactor to produce Pu²³⁹ in the seed andU²³³ in the blanket, continuing such operation of a seed-and-blanketreactor incorporating an optimum proportion of Pu²³⁹ therein, recoveringand retaining additional amounts of Pu²³⁹ produced, recovering andretaining U²³³ so produced until enough is obtained to constitute thefissile fuel loading for a U²³³ fueled light-water breeder reactor.